Microporous crystalline materials
Zeolites are microporous crystalline materials of variable composition characterized by a TO4 tetrahedra crystalline lattice (wherein T represents atoms in the formal oxidation state of +3 or +4, such as for example Si, Ti, Al, Ge, B, Ga . . . ) which all share their vertexes giving rise to a three-dimensional structure containing channels and/or cavities of molecular dimensions. When some of the atoms T have an oxidation state lower than +4, the crystalline lattice formed has negative charges which are compensated by the presence of organic or inorganic cations in the channels or cavities. Organic molecules and H2O may also be located in said channels and cavities, so in general, the chemical composition of zeolites can be represented by the following empirical formula:
X (M1/nXO2):yYO2:zR:wH2O
wherein M is one or several organic or inorganic cations with charge +n; X is one or several trivalent elements; Y is one or several tetravalent elements, generally Si; and R is one or several organic substances. Although the nature of M, X, Y and R and the values of x, y, z, and w may, in general, be varied by means of post-synthesis treatments, the chemical composition of a zeolite (just as it is synthesized or after calcination thereof) has a range characteristic of each zeolite and its preparation method.
On the other hand, a zeolite is also characterized by its crystalline structure, which defines a system of channels and cavities and gives rise to a specific X-ray diffraction pattern. In this way, zeolites are differentiated from each other by their range of chemical composition plus their X-ray diffraction pattern. Both characteristics (crystalline structure and chemical composition) also determine the physicochemical properties of each zeolite and the applicability thereof in different industrial processes.
The present invention refers to a microporous crystalline material of zeolitic nature named ITQ-7, to its method of preparation and to its applications.
The material is characterized by its chemical composition and by its X-ray diffraction pattern. In its anhydrous and calcined form, the chemical composition of ITQ-7 may be represented by the empirical formula:
x(M1/nXO2):yYO2:SiO2
wherein x has a value lower than 0.06, it may be equal to zero; y has a value lower than 0.1; it may also be equal to zero; M is H+ or an inorganic cation of charge +n; X is a chemical element with oxidation state +3 (such as, for example, Al, Ga, B, Cr) and Y is a chemical element with oxidation state +4 (such as, for example, Ti, Ge, V). When x=0 and y=0, the material may be described as a new polymorphic form of the silica (SiO2) characterized by its microporous structure. In a preferred embodiment of the present invention, ITQ-7 has the composition, in a calcined and anhydrous state
x(HXO2):SiO2
wherein X is a trivalent element and x has a value lower than 0.06 and may be equal to zero, in which case the material may be described by the formula SiO2. The existence of defects in the crystalline network, that is manifested by the presence of Sixe2x80x94OH groups (silanols) is possible, however, in terms of the synthesis method and its calcination or subsequent treatments. These defects have not been included in the preceding empirical formulae. In a preferred embodiment of the present invention, ITQ-7 has a very low concentration of this type of defect (silanol concentration lower than 15% with respect to the total Si atoms, preferably lower than 6%, measured by nuclear magnetic resonance spectroscopy of 29Si in magic angle).
The positions, widths and relative intensities of the peaks depend to a certain degree on the chemical composition of the material (the pattern represented in Table I refers to materials whose lattice is exclusively comprised of silicon oxide, SiO2 and synthesized using a quaternary ammonium cation as a structure-directing agent) and they may also be affected by structural alterations such as intergrowths, macles and stacking defects. The relative intensities may also be affected by phenomena of preferred orientation of the crystals, produced during the preparation of the sample, while the precision in the interplanar spacing measurement depends on the quality of alignment of the goniometer. Moreover, calcination gives rise to significant changes in the X-ray diffraction pattern, due to the removal of organic compounds retained during synthesis in the zeolite pores, so that Table II represents the diffraction pattern of calcined ITQ-7 with a composition SiO2.
Furthermore, the relative intensities of the peaks as well as their widths may be affected by the phenomena of preferred orientation and differences of crystal size, as well as by structural alterations such as macles, intergrowths and stacking defects. These differences are illustrated in FIG. 1, wherein the diffraction patterns corresponding to calcined ITQ-7 samples with a composition SiO2 prepared under different conditions are shown.
From the point of view of its chemical composition, ITQ-7 is characterized by having a (Si+Y)/X ratio higher than 8, wherein the X element may be constituted exclusively by Al, and by its low concentration of connectivity defects ( less than 15%, preferably  less than 6%). Furthermore, ITQ-7 may be synthesized without Al, or another element with oxidation state +3, in which case ITQ-7 is a new polymorphic form of silica of microporous nature.
The present invention also refers to the method of preparation of ITQ-7. This comprises thermal treatment at a temperature between 80 and 200xc2x0 C., preferably between 130 and 180xc2x0 C., of a reaction mixture that contains a source of SiO2 (such as, for example, tetraethylorthosilicate, colloidal silica, amorphous silica), an organic cation in hydroxide form, preferably 1,3,3-trimethyltricyclo-6-azonium-[3.2.1.46.6]dodecane hydroxide (I), hydrofluoric acid and water. Alternatively, it is possible to use the organic cation in the form of a salt (for example, a halide, preferably chloride or bromide) and to substitute the hydrofluoric acid by a fluorine salt, preferably NH4F. The reaction mixture is characterized by its relative low pH, pH less than 12, preferably pH less than 10 and it may also be neutral or slightly acidic. 
Cation I has two asymmetric carbons and may be used either as a racemic mixture or as any of its two enantiomers or as mixtures of both enantiomers.
Optionally it is possible to add a source of another tetravalent element Y and/or trivalent element X, preferably Ti or Al. The addition of this element may be done before heating the reaction mixture or in an intermediate time during said heating. Occasionally, it may also be convenient to add at a certain time of the preparation ITQ-7 crystals (up to 15% by weight with respect to the total inorganic oxides, preferably less than 10% by weight) as crystallization promoters (seeding). The composition of the reaction mixture responds to the general empirical formula
rR2O:aHF:xXO2:yYO2:SiO2:wH2O
wherein X is one or several trivalent elements, preferably Al; Y is one or several tetravalent elements; R is an organic cation, preferably 1,3,3-trimethyltricyclo-6-azonium-[3.2.1.46.6]dodecane; and the values of r, a, x, y and w are in the ranges
r=0.05-4.0, preferably 0.1-3.0
a=0-4.0, preferably 0.1-3.0
x=0-0.12
y=0-0.5
w=0-100, preferably 1-50, more preferably 1-15
The thermal treatment of the reaction mixture may be done in static or with stirring of the mixture. Once the crystallization has finished, the solid product is separated and dried. Subsequent calcination at temperatures between 400 and 650xc2x0 C., preferably between 450 and 600xc2x0 C., produces the decomposition of the organic residues occluded in the zeolite and the removal thereof and of the fluoride anion, rendering the zeolitic channels free.
This method of synthesis of ITQ-7 zeolite has the peculiarity that it does not require the introduction of alkali cations in the reaction medium. As a result the organic cation R is the only cation that balances the lattice charges when the zeolite contains a trivalent element in its crystalline lattice. Therefore, simple calcination in order to decompose the organic cation leaves the zeolite in acid form, without the need to resort to cation exchange processes. Therefore, the material once calcined responds to the general formula
x(HXO2):yYO2:SiO2
wherein x has a value lower than 0.06, it may be equal to zero; y has a value lower than 0.1, it may likewise be zero; X is a chemical element with oxidation state +3 and Y is a chemical element with oxidation +4.